HP38 comes off the same production line in St Marks, Florida as Win 231.

BUT, I have seen both powders in the same manual with slightly different top loads and velocities.
And I GET slightly different velocities from the same charge of HP38 and Win231. HP38 runs a little faster than Win 231 in .38, 9mm, and .45.

Now that might be lot-to-lot variation, or maybe Hodgdon specified powder to be sold as HP38 a little different from what Winchester sells as Win 231.
Or maybe the first batch Hodgdon got was a little faster than what Winchester was selling, and for their next and present orders they have said "Send me more like THAT." A combination of lot variations and company specs.

Moral of the story: Use brand-specific load data and refine with a chronograph if available.

Not that Mr. Watson needs back-up. As I understand it, knowing the difference between certain pistol powders is a requirement for graduating High School in his neck-of-the-woods. If it is not it should be. The country would be better if they did.

Depend on chemical "deterrents" to control burning rate. Different powders can come off the same production line, what varies is the chemical make up and treatment of the batch.

231 and HP 38 are listed as different powders, and so they are. No matter how similar they may appear, different powders are well, different. Use only the data for the specific powder you are using, and work up your loads using safe reloading practices.

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All else being equal (and it almost never is) bigger bullets tend to work better.

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